OFDM systems are well known. Various techniques have been used for synchronisation of OFDM receivers. Some of these techniques require transmission of a special synchronisation signal. Other techniques rely on a standard OFDM signal, in which a complete symbol comprises a “useful part” and a “guard space”, the guard space sometimes being referred to as a guard interval, cyclic extension or cyclic prefix.
The guard space precedes the useful part of the symbol and contains a repeat of the data at the end of the useful part. (This is equivalent to having a guard space after the useful part, containing data which is the same as that at the beginning of the useful part.)
Synchronisation techniques which rely upon the duplicated data in the guard space generally operate by performing a cross correlation between complex samples spaced apart by the length of the useful part of the symbol. This generates a timing pulse which is used in Fourier Transformation of the received signal. The timing of the pulse is such that the Fourier Transform window contains only data from a single symbol.
If the timing is incorrect, inter-symbol-interference (ISI) occurs. However, the use of the guard space allows a certain amount of variation in the timing of the pulse while still avoiding ISI. The guard space should be longer than the longest expected spread of delays amongst signals received via different paths. The guard space is relatively small compared with the useful part of the signal; typically, the guard space may contain Nu/32, Nu/16, Nu/8 or Nu/4 samples, where Nu is the number of samples in the useful part of the symbol.
Various techniques exist for deriving the synchronisation pulse from the cross-correlation. Although these operate adequately in common reception conditions, there are circumstances in which the timing pulse is generated at an undesirable point, leading to ISI.
The cross-correlator, in the absence of noise or multi-path interference, produces an output which averages to zero except during the time that the guard space samples are cross correlated with the samples, in the useful part of the symbol, which are of equal value. During that period, the cross-correlator produces a high-level output. This high-level output terminates at the end of one symbol and the beginning of the next. One prior art arrangement integrates the output of the correlator, and then peak-detects the resultant signal to produce a timing pulse at the end of each symbol.
In the case of multi-path interference, wherein the same signal is received via different delays, in order to avoid ISI, the synchronisation pulse should be generated during a window which has a width equal to the overlap between the guard spaces of the two received signals. However, the cross-correlator will produce a significant output throughout the period in which samples of either one or both of the guard space samples are being processed by the cross-correlator. In some circumstances, this will result in the timing pulse being provided outside the optimum window, thus resulting in ISI.
EP-A-0 772 332 describes other techniques for generating a synchronisation pulse. One such disclosed technique relies upon feeding the output of the correlator to a phase locked loop (PLL). This can also result in the synchronisation pulse being generated outside the optimum window in the case of significant noise or multi-path interference. Furthermore, a PLL requires a substantial number of symbol periods in order to achieve lock, which therefore results in substantial acquisition time.
A further problem which can arise in prior art arrangements results from the fact that, when the synchronisation pulse is adjusted as a result of, for example, changing signal conditions, the complex values in the frequency bins at the output of the FFT suffer varying degrees of phase rotation. Although a subsequent channel estimator and corrector can handle these changes, this can result in a further increase of acquisition time and requires a significant amount of processing power.
It would therefore be desirable to provide a technique for generating a synchronisation pulse in which these problems are avoided or at least mitigated.